Organometallic Compounds - Organic Pollutants: An Ecotoxicological Perspective - page 163

8 Organometallic

Compounds

8.1 Background
Metalloids such as arsenic and antimony, and metals such as mercury, lead, and
tin—which occupy a similar location to metalloids in the periodic system—all tend
to form stable covalent bonds with organic groups. Some authorities regard tin as a
metalloid. By contrast, metals such as sodium, potassium, calcium, strontium, and
barium, which belong to groups 1 and 2 of the periodic system, do not form cova-
lent bonds with organic groups. The compounds used as examples here all possess
covalent linkages between a metal and an organic group—most commonly an alkyl
group. The elements in question are mercury, tin, lead, and arsenic, all of which are
appreciably toxic in their inorganic forms as well as in their organometallic forms.
The attachment of the organic group to the metal can bring fundamental changes in
its chemical properties, and consequently in its environmental fate and toxic action.
In particular, the attachment of alkyl or other nonpolar groups to metals increases
lipophilicity and thereby enhances movement into and across biological membranes,
storage in fat depots, and adsorption by the colloids of soils and sediments. Thus, the
question of speciation is critical to understanding the ecotoxicology of these metals.
In the first place, organometallic compounds of mercury, tin, lead, and arsenic
have been produced commercially, mainly for use as pesticides, biocides, or bacte-
ricides. Additionally, methyl mercury and methyl arsenic are generated from their
inorganic forms in the environment, so residues of them may be both anthropogenic
and natural in origin. Most of the following account will be devoted to organomer-
cury and organotin compounds, which have been extensively studied. Organolead
and organoarsenic compounds have received less attention from an ecotoxicological
point of view, and will be dealt with only briefly.

8.2 organomercury comPoundS

8.2.1 o r i g i n s a n d c h e m i c a l p r o p e r T i e s
A range of organomercury compounds have been produced commercially since early
in the 20th century, principally for use as antifungal agents. Most of them have the gen-
eral formula R-Hg-X, where R is an organic group and X is usually an inorganic group
(occasionally a polar organic group such as acetate). The organic group is nonpolar (or
relatively so) and gives the molecule a lipophilic character. The most common organic
groups are alkyl, phenyl, and methoxyethyl (see Environmental Health Criteria 86).

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